1. Field of the Invention
The present invention relates generally to microwave antennas and, more particularly, to a waveguide fed antenna which can produce independent, selectable transverse and longitudinal polarizations. The invention is useful in providing polarization control in antenna arrays.
2. Description of Related Art
In antenna array applications for microwave communications and radar applications, for example, it is desirable to be able to control the polarization of the antenna with respect to the longitudinal and transverse planes. In a waveguide slot array antenna, it is often impractical to move or tilt the radiating elements. Hence, the radiation polarization must be controlled by controlling the phase and/or amplitude of the excitation energy being delivered to each of two orthogonal sets of radiating elements. In waveguide slot array antennas, the radiating elements are in the form of slots or apertures positioned at spaced intervals (usually a half waveguide wavelength apart). In some waveguide array designs, a plurality of such slotted waveguides are stacked with their respective slotted walls in a common plane to define a two-dimensional matrix configuration.
With such slot array antennas of the prior art, it has been difficult to select between longitudinal and transverse polarization (perpendicular to the waveguide length). The slots which comprise such arrays naturally produce a linear polarization which aligns with the transverse dimension of the slot. Thus, a prior art slot array of conventional configuration produces primarily a single polarization, dictated by the orientation of the slots.
One possible technique for providing dual polarization is described in U.S. Pat. No. 3,503,073 to Ajioka, entitled "Two-Mode Waveguide Slot Array", which is assigned to the assignee of the present invention. That technique employs a waveguide which includes an internal septum wall extending partially across the interior of the waveguide and which has a plurality of non-resonant crossed slots. This prior art configuration is best used in traveling wave-fed arrays, where the susceptance of the non-resonant slots is tolerable. This technique is not well suited for standing wave-fed arrays, such as flat plate antenna arrays, however. Another disadvantage of the crossed-slot configuration is that the required dimensions of the slots limit the element spacing in the transverse plane to something larger than could be achieved with parallel slots (non-crossed slots). The ability to closely space the aperture elements is desirable since it allows a wider scan angle to be achieved before grating lobes appear.